[IOT-1602] Remove observer when the tcp connection is disconnected

[IOT-1602] Remove observer when the tcp connection is disconnected

If the client observes one or more resources over a reliable
connection, then the CoAP server (or intermediary in the role of the
CoAP server) MUST remove all entries associated with the client
endpoint from the lists of observers when the connection is either
closed or times out.
Bug: https://jira.iotivity.org/browse/IOT-1602

Change-Id: I31dcf26a3dc731b4479c9d7a7a4755c7afd07ff8
Signed-off-by: hyuna0213.jo <hyuna0213.jo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14683
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jaewook Jung <jw0213.jung@samsung.com>
Reviewed-by: Ziran Sun <ziran.sun@samsung.com>
(cherry picked from commit 76ac4cfd9ffb58a17c93d74c3f6a21b11b07cf50)
Reviewed-on: https://gerrit.iotivity.org/gerrit/15147
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Reviewed-by: Jaehong Jo <jaehong.jo@samsung.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>